Specific tetrahydrocannabinol antisera are utilized in radioimmune experiments to detect in vivo and quantitate in vitro tetrahydrocannabinol and its metabolites Standard antibody binding curves and inhibitions of marker bindings to antibody by native THC metabolites have been established. Clinical sample analyses are in progress. Clincial experiments have included active immunization of rats and subsequent challenge with native THC to measure in vivo competition of fixed receptors and antibodies for THC.